With popularity of smart terminals and continuous development of demands on mobile new services, requirements on wireless transmission rate increases exponentially. As a result, when the four-generation (4G) mobile communication technologies are being applied, such as Long-Term Evolution (LTE) system, research on the five-generation (5G) mobile communication technologies has started. Compared with 4G, 5G should meet several key requirements, for example, 1) transmission rate is improved 10˜100 times, experience rate to end users (UEs) is up to 1 Gb/s, and user peak rate is up to and beyond 10 Gb/s; 2) the density of connected devices is improved 10˜100 times, up to millions per square kilometer. In order to fulfill the above-described requirements, and considering the limited spectrum resources which is available up till now, during research work on 5G, one method is to propose massive MIMO and use spectrum resources at high frequency, so as to further explore spatial dimensional radio resources and solve the problem about spectrum efficiency and power efficiency in future mobile communication.
Features of massive MIMO wireless communication technologies include: deploying tens of or hundreds of antennas in the coverage of base station, and placing those antennas in a massive array. Multiple users distributed in the coverage of base station communicate with the base station simultaneously on a same time and frequency resource by taking advantage of spatial freedom provided by the massive antennas configured at the base station, which can improve the multiplexing capability of spectrum resources among multiple users, spectral efficiency of each user's link and capability to suppress the inter-cell interference, so that the overall efficiency of spectrum resources may be enhanced greatly. In addition, by using the diversity gain and array gain provided by the massive antennas configured at the base station, the power efficiency of the communication link between each user and the base station may be improved further.
The wavelength of a high-frequency signal is short, so the high-frequency band is suitable for massive MIMO with densely deployed antenna elements. But the high-frequency signal tends to be more influenced by path loss, and a bottleneck caused by the significant increase of antenna number only at the base station occurs for the massive MIMO technologies. Thus, it can be seen that, massive MIMO is different from the current MIMO transmission technologies. It is required to research on the wireless transmission technologies which is feasible in the scenario of massive MIMO.
On the other hand, in massive MIMO, if one Radio Frequency (RF) chain is installed for each antenna element, complexity, power consumption and cost are increased. In this way, hybrid beamforming can enable multiple antenna elements to use one RF chain, thus it becomes a research topic as a low cost and feasible solution for massive MIMO.